1. Technical Field
The present invention relates to a rich-lean burner having on an upper end portion thereof, a lean flame port extended in a back-and-forth direction (in a meaning of “longitudinal direction” or “longitudinally”) to eject a lean fuel-air mixture which is leaner in fuel concentration than a theoretical fuel-air ratio; a blind clearance which is located on each side in the right-and-left direction (in a meaning of “lateral direction” or “laterally”) of the lean flame port and which does not eject (or which is free from ejection of) a fuel-air mixture; and a rich flame port located on lateral outside of each of the blind clearances thereby ejecting a rich fuel-air mixture which is richer in fuel concentration than the theoretical fuel-air ratio.
2. Background Art
Although this kind of rich-lean burner is capable of reducing the NOx produced at the time of combustion, oscillated combustion is likely to occur at the time of high load combustion, thereby giving rise to the problem of combustion noises. As a solution, there is known a rich-lean burner in which the lean flame port is divided into a plurality of longitudinally disposed blocks so that the fuel concentrations of lean fuel-air mixture fluctuate within each of the blocks (see, e.g., Japan Patent No. 3614745). This prior art says that the frequency of combustion oscillation that occurs by the combustion of lean fuel-air mixture ejected from the lean flame port, is dispersed due to fluctuations in fuel concentrations, thereby suppressing the oscillatory combustion.
However, when the fuel concentrations in the lean fuel-air mixture get fluctuated as in the above-described prior art, there are disadvantages in: that the combustibility is likely to be deteriorated; that the region of good combustion (the range in which the combustion can take place well) becomes narrower; and that the margin becomes smaller for the change in volume of gas or for the change in volume of combustion air to be blown to the burner.